Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
A typical printing plate precursor for computer-to-film methods comprise a hydrophilic support and an image-recording layer of a photosensitive polymer layers which include UV-sensitive diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used. Upon image-wise exposure, typically by means of a film mask in a UV contact frame, the exposed image areas become insoluble and the unexposed areas remain soluble in an aqueous alkaline developer. The plate is then processed with the developer to remove the diazonium salt or diazo resin in the unexposed areas. So the exposed areas define the image areas (printing areas) of the printing master, and such printing plate precursors are therefore called ‘negative-working’. Also positive-working materials, wherein the exposed areas define the non-printing areas, are known, e.g. plates having a novolac/naphtoquinone-diazide coating which dissolves in the developer only at exposed areas.
In addition to the above photosensitive materials, also heat-sensitive printing plate precursors have become very popular. Such thermal materials offer the advantage of daylight-stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilisation by cross-linking of a polymer, heat-induced solubilisation, decomposition, or particle coagulation of a thermoplastic polymer latex.
The known heat-sensitive printing plate precursors typically comprise a hydrophilic support and a coating containing an oleophilic polymer, which is alkali-soluble in exposed areas (positive working material) or in non-exposed areas (negative working material) and an IR-absorbing compound. Such an oleophilic polymer is typically a phenolic resin.
WO99/01795 describes a method for preparing a positive working resist pattern on a substrate wherein the coating composition comprises a polymeric substance having functional groups such that the functionalised polymeric substance has the property that it is developer insoluble prior to delivery of radiation and developer soluble thereafter. Suitable functional groups are known to favor hydrogen bonding and may comprise amino, amido, chloro, fluoro, carbonyl, sulphinyl and sulphonyl groups and these groups are bonded to the polymeric substance by an esterification reaction with the phenolic hydroxy group to form a resin ester.
EP-A 0 934 822 describes a photosensitive composition for a lithographic printing plate wherein the composition contains an alkali-soluble resin having phenolic hydroxyl groups and of which at least some of the phenolic hydroxyl groups are esterified by a sulphonic acid or a carboxylic acid compound.
EP-A 1 072 432 describes an image forming material which comprises a recording layer which is formed of a composition whose solubility in water or in an alkali aqueous solution is altered by the effects of light or heat. This recording layer comprises a polymer of vinyl phenol or a phenolic polymer, wherein hydroxy groups and alkoxy groups are directly linked to the aromatic hydrocarbon ring. The alkoxy group is composed of 20 or less carbon atoms.
U.S. Pat. No. 3,929,488 describes a light sensitive positive-working printing plate comprising an azo-dye which is a novolac resin, bearing diazophenyl chromophoric moiety on the phenolic ring, and which undergoes a color change in the presence of the light decomposition product of a diazonium salt. This color change is caused by a protonation of the azo-dye nitrogens by the acid produced by the diazonium salt on exposure to light. This azo-dye is used as an indicator dye which allows to inspect the light-struck areas on the plate. In comparison with conventional indicator dyes, this azo-dye has an increased solubility in the developer solution and, consequently, is not left behind after development on the plate. As a result, there is no risk of remaining dye, which can cause staining on the non-image areas or scumming during the printing process. The most favored azo-dye as indicator dye in these light-sensitive compositions which can undergo such a strong color change in the visible light range by protonation, is an azo-dye, derived of an azoic coupling of diphenylamine-4-diazonium fluoroborate with novolac.
The ink and fountain solution which are supplied to the plate during the printing process, may attack the coating and, consequently, the resistance of the coating against these liquids, hereinafter referred to as “chemical resistance”, may affect the printing run length. The most widely used polymers in these coatings are phenolic resins and it has been found in the above prior art that the printing run length can be improved by modifying such resins by a chemical substitution reaction on the hydroxyl group of the phenolic group. However, this modification reaction decreases the number of free hydroxyl groups on the polymer and thereby reduces the solubility of the coating in an alkaline developer. The modification reaction proposed in the present invention enables to increase the chemical resistance of the coating without substantially reducing the developability of the coating.